Evaluating washing and sifting methods for sampling gastropods in freshwater vegetation samples

Abstract

AbstractEuropean freshwater habitats are experiencing degradation and fragmentation, and this is causing population declines and range reductions in associated gastropod species. Accurate and time-efficient sampling methods are required for effective monitoring of such species, including Segmentina nitida, a priority species in the UK Biodiversity Action Plan. The diminutive size of S. nitida (adult shell width: 5–7 mm) and other small gastropods often makes them difficult to find among the vegetation in field samples. Here, we compare two sampling methods for gastropods from freshwater habitats with dense vegetation: a sift method where vegetation in a sample is examined by hand, and a wash method, where vegetation is washed in water that is then examined for gastropods. When sift and wash methods were compared for a single sample, there was no difference in the number of species detected with the two methods in all but 1 of 48 assessed samples. However, combining two wash steps and a sifting step yielded significantly more snail individuals and more S. nitida individuals. Though species richness as expressed by Menhinick’s index was significantly higher for the wash method compared to the sift method, index values for the two methods had a significant linear relationship. Splitting samples to assess each half with either wash or sift method indicated no significant difference in the number of species or number of individuals found with the two methods. The likelihood of detecting S. nitida in a sample and the number of S. nitida found was significantly higher for the wash method, and the wash method was significantly more time-efficient, saving c. 5 min per sample. We conclude that washing vegetation from freshwater samples without sifting through the vegetation by hand has the potential to accelerate surveys for freshwater gastropods and can increase the likelihood of detecting species of interest, especially S. nitida, without significantly affecting data accuracy. Further testing in various freshwater habitat conditions would allow wider use of this method in malacological and invertebrate studies.